The present invention relates to an active color liquid crystal display element which includes in a liquid crystal cell pixel or display electrodes, thin film transistors for charging and discharging them, and color filters of three colors disposed opposite the display electrodes, respectively, and distributed substantially uniformly throughout the liquid crystal cell.
A conventional liquid crystal display element of this kind has such a structure as shown in FIG. 1, in which liquid crystal 14 is sealed in a space defined by a pair of opposed transparent substrates 11 and 12 as of glass which are separated by a spacer 13 interposed between their marginal edges. On the inside surface of the one transparent substrate 11 a plurality of display electrodes 15 are arranged in a matrix form and a plurality of thin film transistors 16 are each disposed in contact with one of them, as a switching element therefor. Each thin film transistor 16 has its drain connected to the display electrode 15 corresponding thereto. The other transparent substrate 12 is covered substantially all over the entire area of its inner surface with a common transparent electrode 17 in opposing relation to the plurality of display electrodes 15.
As shown in FIG. 2, the display electrodes 15 substantially square in shape are disposed close in rows and columns on the transparent substrate 11, a gate bus 18 is formed along each row of the display electrodes 15 in adjacent but spaced relation thereto, and a source bus 19 is similarly formed along each column of the display electrodes 15 in adjacent but spaced relation thereto. At each intersection of the gate and source buses 18 and 19 the thin film transistor 16 is disposed with its gate connected to the gate bus 18, its source connected to the source bus 19 and its drain connected to the corresponding display electrode 15.
Voltage is applied across a selected one of each of the gate and source buses 18 and 19, by which is conducted only one of the thin film transistor 16 that is supplied with the voltage, and charges are stored in the display electrode 15 connected to the drain of the conducted thin film transistor 16. Thus, voltage is applied across the liquid crystal 14 only between the charged display electrode 15 and the common electrode 17, by which only that portion of the liquid crystal 14 is made transparent or untransparent to light, thus providing a selective display. The display can be erased simply by discharging the stored charges from the display electrode 15.
Conventionally, the thin film transistor 16 has such a structure as depicted in FIGS. 3 and 4. That is, the display electrode 15 and the source bus 19 are each formed by a transparent conductive film as of ITO on the transparent substrate 11, a semiconductor layer 21 of amorphous silicon or similar material is formed between and over opposed marginal edges of the display electrode 15 and the source bus 19 lengthwise thereof, and a gate insulating film 22 of silicon nitride or the like is formed over the semiconductor layer 21. On the gate insulating film 22 a gate electrode 23 is formed in an overlapping relation to the semiconductor layer 21 between the display electrode 15 and the source bus 19. The gate electrode 23 is connected at one end to the gate bus 18. The display electrode 15 and the source bus 19 thus lying opposite the gate electrode 23 constitute drain and source electrodes 15a and 19a, respectively. The electrodes 15a and 19a, the semiconductor layer 21, the gate insulating film 22, and the gate electrode 23 make up the thin film transistor 16. The gate electrode 23 and the gate bus 18 are simultaneously formed of aluminum, for instance.
On the transparent substrate 12 red, green and blue color filters 1R, 1G and 1B are each formed opposite one of the display electrodes 15. These color filters are substantially uniformly distributed so that they are fairly intermingled with one another, as shown in FIG. 3. In this way, an active color liquid crystal display element is constituted.
Incidentally, a color liquid crystal display element which utilizes such color filters and TN liquid crystal, for example, shows different light transmission characteristics for red (R), green (G) and blue (B) color signal voltages, as depicted in FIG. 5. This does not yield a color display of good quality. A solution to this problem, proposed so far, is a multigap structure in which the thickness of the liquid crystal thickwise of the liquid crystal cell is changed for each color filter, thereby compensating for each color signal voltage-transmission characteristic, as disclosed in Japanese Patent Application Laid Open No. 159823/85.
The color liquid crystal display element of this multigap structure encounters difficulty in controlling the thicknesses of the color filters and suffers liability to variations in orientation owing to irregularities of the color filters. Furthermore, the varying thicknesses of the liquid crystal yield different response times for red, green and blue color signals.